a special pair of chromosomes containing the genes that determine the sex found in the chromosome bank of dioecious organisms. Differences were discovered in the chromosome banks of male and female insects and unusual sex chromosomes were described by the German researcher H. Henking in 1891 and by C. McClung and E. Wilson in the early 20th century. It was later discovered that many dioecious organisms have sex chromosomes containing sex-determining elements.

Usually, chromosomes constitute an unequal pair. The larger chromosome contains the female sex factors and is called the X chromosome; the smaller chromosome is called the Y chromosome. In mammals, including man, the factors that determine the male sex are localized in the Y chromosome. In other organisms, for example, in Drosophila, the Y chromosome is contained in autosomes. In many species the Y chromosome is absent when the male sex is determined by autosomes.

Females usually have two identical sex chromosomes (XX type) and males have either two unequal chromosomes (XY type) or a single sex chromosome (X0 type). Females are said to be homogametic because, as a result of meiosis, each egg contains one X chromosome. Males are heterogametic because two types of sperm, containing either the X chromosome or the Y chromosome, are formed from the XY pair. The chance union of sex cells, or gametes, during fertilization results, on a large scale, in an equal number of females (XX) and males (XY).

The opposite relationships are true in butterflies, birds, some reptiles, and amphibians. The males are homogametic and possess the XX chromosome. Accordingly, a single type of sperm with an X chromosome is formed. The females are heterogametic and have XY chromosomes. As a result, two types of eggs are formed: one with an X chromosome and the other with a Y chromosome. Chromosomes are generally designated by Z and W when females are heterogametic.

In addition to the genes that determine sex, sex chromosomes also contain sex-linked genes that determine an organism’s different characters and are inherited with sex. There are many sex-linked genes in an X chromosome and only a few in a Y chromosome. In man the recessive genes of hemophilia and albinism are examples of sex-linked genes. These genes manifest themselves exclusively in males if they are present in only one of the X chromosomes of the female. Females are then the carriers of latent sex-linked diseases.

Deviations from the normal number of sex chromosomes in human cells result in developmental anomalies. Shereshevskii-Turner’s syndrome occurs in women with an XO chromosome and is characterized by short stature, infertility, and mental retardation. Klinefelter’s syndrome occurs in men with an XXY chromosome and is characterized by excessive height, long extremities, abnormal development of sex characters, infertility, and mental retardation. As many as four X chromosomes can be paired with a Y chromosome (XXXXY) in those with this syndrome. Trisomy syndrome occurs in women with an extra X chromosome (XXX) and is characterized by mental retardation and underdevelopment of the ovaries.

The Y chromosome is easily visible in cells because part of it can be selectively stained with fluorescent dyes of the quinacrine hydrochloride variety; this procedure is used for diagnostic purposes.

Sex chromosomes are found in some diclinous plants; for example, in strawberries. Hermaphroditic animals and monocli-nous plants do not have sex chromosomes.